1. Field of the Invention
The invention relates generally to an underwater acoustic reflector for use with a hydrophone assembly and, more particularly, to an acoustic baffle providing high compliance in a liquid medium under high hydrostatic pressure.
2. Description of the Prior Art
It is well known that self-noise resulting from noise generating mechanisms such as machinery and induced vibrations in the hull structure of a submarine, reduce the signal to background noise ratio in a hydrophone array. Acoustic baffles, either of the reflective or absorptive type, have been used with sonar arrays to discriminate against such noise sources. Sound energy impinging on a compliant (or low impedance) baffle layer mounted behind a hydrophone array and outboard the hull of a vessel will cause energy on the submarine side to be reflected back away from the hydrophones because of the impedance discontinuity. However, baffles which are too highly compliant, while yielding good shallow depth performance, can quickly "bottom out" and thereby lose their effectiveness at medium depths.
Common types of wholly compliant baffles consist of rubber tiles with various size air pockets. Under increasing hydrostatic pressure, the soft walls of the air pocket enclosures buckle and pocket coverings deflect quickly, reaching a bottoming or non-operational condition at a shallow ocean depth. Making the linings of the pockets stiffer or adding particles of lead shot to the pockets provide some increase in operational depth capability, although limited.
Other methods, such as inclusion of springs within the rubber compound to limit overall compliance and extend performance to reasonable depths have also been attempted, as in U.S. Pat. No. 3,277,434. In U.S. Pat. No. 3,907,062, a compliant baffle provided with an acoustically harder outer surface material had embedded therein a plurality of tubes capable of withstanding the required hydrostatic pressure without collapse, but compliant to the noise frequencies encountered. However, this design has several shortcomings. The tubes are tuned to resonate and because they have little inherent damping they yield non-uniform noise reduction, being effective only in a limited frequency band. Staggering tubes of different sizes and resonant frequencies in horizontal and vertical arrays, while providing a broader frequency band performance, does not alleviate the non-uniformity of response problem due to the limited damping.
A further problem with the above cited art is that the compliance mechanism of the embedded tubes is equivalent to that of a series of beams with both ends fixed, with the beam deflecting under application of uniform pressure loading. This fixed end design is susceptible to high stress concentrations at the ends, which tends to result in early fatigue failure when subjected to cyclic pressure stressing. Further, since such tubes are likely to be fabricated of a non-metallic material such as fiberglass or carbon reinforced plastic laminate for minimizing weight (of critical importance in submarine array design), the tubes must be greatly derated with respect to the allowable stress loading, thereby limiting the operational depth of the baffle. Moreover, the nature of this arrangement is such that the tubes do not bottom, but continue to deflect and are therefore subjected to increased bending stress when loaded to a test pressure or depth (which may be greater than the maximum operational pressure), thus further limiting the utility for deep depth operation at high hydrostatic pressures.
The present invention utilizes a highly compliant polymer in which are embedded a plurality of damped spring elements having a predetermined maximum deflection, thereby providing high acoustic efficiency while resisting pressures beyond the operational depth of the hydrophone array.